1. Field of the Invention
This invention relates generally to light emitting diodes (LEDs) used in light sources in automobiles and other vehicles, and in particular to a driver circuit that provides linear regulated current and reduced input voltage operation.
2. Description of Related Art
Advances in light emitting diodes (LEDs) have made LEDs very attractive for use in vehicles because of their long operation life, higher efficiency and low profile. LED light output is proportional to the LED current, and therefore, a current source is the preferred method of driving the LEDs. When LEDs are used in higher power applications such as the rear combination lights (STOP/TURN/TAIL), lower efficiency, cumbersome current regulation, switching noise and cost have been problems along with a desire to operate over a reduced input voltage range.
Prior art patents include U.S. Pat. No. 5,742,133 issued Apr. 21, 1998 to Wilhelm Wilhelm et al and assigned to Siemens Aktiengesellschaft of Munich, Germany discloses a driver circuit for a LED comprising a switch device connected to a LED controlled by an input signal and having a current source. The switch device short-circuits the LED after a transition to a first switching state, and the switch device supplies the LED from the current source after a transition to a second switching state. However, this switching driver circuit produces unwanted switching noise and does not have a TAIL light capability.
U.S. Pat. No. 6,362,578 issued Mar. 26, 2002 to David F. Swanson et al and assigned to STMicroelectronics, Inc. of Carrollton, Tex. which discloses a LED driver circuit having a PWM controller for setting a PWM duty cycle for LED arrays of light emitting diodes (LEDs) to determine the brightness of the LEDs. However, hard switching of the LEDs ON and OFF occurs which causes switching noise to be generated and the switching noise makes it difficult to meet automotive requirements for radiated and conducted emissions. U.S. Pat. No. 6,586,890 issued Jul. 1, 2003 to Young-Kee Min et al and assigned to Koninklijke Philips Electronics, N.V. of Eindhoven, NL discloses a LED driver circuit for providing power to LEDs using pulse width modulation (PWM). Current feedback is used to adjust power to LED arrays and provides a full light and dim light modes of operation. A frequency is selected to limit electromagnetic interference (EMI). However, a power supply must be provided to account for variation in the input voltage to keep the string of LED's supplied with enough voltage.